I'm installing a shower stall and have been researching faucet options. I'd like to have a regular shower head as well as a "rain fall" one. I know I can buy a separate diverter to use with the 2 devices. My question, why can't I use a valve that has a diverter built into it? The installation instructions say the "tub spout" should be a straight drop with only a 90 and that you shouldn't substitute something for the tub filler. Why is that? I don't have an issue with setting it up properly. I'm just curious about the logic. Is there a "real" reason or is it so you buy another piece?